The present invention describes an operation of compensating for defective pixels in an image sensing device.
Any manufactured device has certain defects. Minimization of defects yields a higher quality product. However, it is usually less expensive to make a device using less precise manufacturing techniques. Devices that are produced using less precise manufacturing techniques have a higher probability of defects.
Typical semiconductor processing techniques define some tradeoff between the quality and cost of manufacture. The semiconductors are tested for defects, and any semiconductor which has more than a certain percentage of defects is usually discarded.
Optical semiconductors, and especially image acquisition semiconductors, are especially sensitive to defects. A bad area in such a semiconductor will show up as a bad area on the image that is acquired by the semiconductor.
Optical devices operate on spatially distributed units of picture elements or pixels. A typical defect in such an optical semiconductor is a pixel errorxe2x80x94xe2x80x9cdead pixelxe2x80x9d. The dead pixels may not work at all, or alternatively may be brighter or dimmer than desired. Depending on the desired quality and the intended application, a single defective pixel may be sufficient to cause the device containg pixels to be discarded.
The inventors recognized the tradeoff between cost of manufacture and percentage of usable product. The present specification defines techniques for compensating for dead pixels in an optical device.
The optical devices are usually formed on a semiconductor substrate. The area of that substrate, or xe2x80x9creal estatexe2x80x9d, is usually quite limited, especially in an image sensing array. Images are acquired by sensing and accumulating incoming optical energy. The available real estate on an image sensing chip is hence predominately consumed by the pixel array.
Active pixel sensors use some of the area of the substrate to house control circuitry. An active pxiel sensor is described in U.S. Pat. No. 5,471,515, the disclosure of which is herein incorporated by reference. These new techniques have opened the way for incorporating associated control circuitry on the chip. However, the inventors recognized that this other required circuitry limits the amount of real estate that can be allocated to the dead pixel correcting system.
A high quality image is often formed of many pixels. Storing a map of an entire 1024xc3x971024 array would require 1024xc3x971024=1 Mb of memory. The inventors recognized that this amount of memory could occupy an impractical percentage of the chip real estate.
According to one aspect, a tradeoff is determined between the amount of memory and hence real estate which will be dedicated to dead pixel correction, and the expected number of dead pixel areas. A practical upper bound on the number of dead pixel areas is defined. That defined number of dead pixel areas can be corrected.
Another aspect of the invention allows a single indication to be stored to represent either a dead pixel or some pre-defined group of dead pixels. A dead pixel area is hence identified as either a pixel or a group of pixels that includes pixel elements that are in some way defective.